Fewer people equals fewer emissions, study finds

LOS ANGELES - Ever since the Industrial Revolution brought belching smokestacks, greenhouse gases and human population have climbed in lockstep to ever-higher levels.

And while scientists warn that humanity must dramatically slash future carbon-dioxide emissions to avert extended droughts, floods and other climate catastrophes, they have generally avoided a rigorous examination of how slowing population growth would help. Now, an international team of scientists has done the math.

If global population were to grow by less than a billion by midcentury, instead of by more than 2 billion, as expected, it would be the equivalent of cutting as much as 29 percent of the emissions reductions needed by 2050 to keep the planet from tipping into a warmer, more dangerous zone. By the end of the century, the effect would be up to 41 percent.

That's similar to some of the other large-scale emissions-cutting strategies, such as doubling the fuel efficiency of 2 billion cars from 30 miles per gallon to 60.

Published today in the Proceedings of the National Academies of Science, the study offers a novel way to quantify how changes in human population influence the buildup of greenhouse gases.

Brian O'Neill, senior scientist at the National Center for Atmospheric Research in Boulder, Colo., was the lead author.

Drawing on surveys from 34 countries representing 61 percent of the world's population, the analysis looks at how demographic changes affect economic growth and energy use and the resulting pollution.

Today's population of 6.9 billion is expected to reach 9.1 billion by 2050, according to UN demographers. They suggest two alternative scenarios: a path of slower growth that would reach slightly less than 8 billion or a faster one that would reach nearly 10.5 billion.

But it's not just population size that matters, the study found. It's also where and how people live.

Growth in the United States has an outsized influence. An average U.S. resident emits four times as much carbon dioxide as a resident of China and 20 times as much as an African. The U.S. Census Bureau estimates that U.S. population of 310 million will swell to 439 million by midcentury, with most of the growth from immigrants and their children.

More broadly, the scientists found the trend toward urbanization had a large effect on pollution levels. In developing countries, a typical farmer may or may not have electricity back home but generates more pollution after moving into a bustling city with modern amenities.